Facet Etching Promises Increased Blue-laser Yield
A problematic procedure
Cleaving GaN is particularly problematic. Nichia Chemical of Japan first used mechanical cleaving to fabricate GaN-based blue lasers on sapphire substrates in 1995 and has since made continuous-wave lasers commercially in the same way. However, the cost of these lasers remains high.
Cleaving sapphire to form GaN-based laser facets is difficult. The substrate has several cleave planes of approximately equal strength that are orientated at such acute angles to one another that minute perturbations occurring during cleaving can redirect a fracture interface from one cleave plane to another. Despite this problem, sapphire s low cost and its stability during the high-temperature growth processes needed for GaN deposition have helped it to remain the substrate of choice for nitride laser fabrication.
Another disadvantage of sapphire and the more expensive SiC substrates is the significant lattice mismatch with GaN that causes the grown layers to have high defect densities. Free-standing GaN substrates could offer a partial solution, but they are only just becoming available. However, unlike InP and GaAs, which have a cubic crystal structure, GaN s structure is hexagonal. This makes GaN much harder to cleave, and it is expected that cleaving will remain a challenging process even if GaN substrates do become the standard.
Several years ago an alternative technology was pioneered at Cornell University, NY. It uses a process based on photolithography and chemically assisted ion-beam etching (CAIBE) to form the facets. BinOptics Corporation, Ithaca, NY, has since developed commercially available InP-based lasers using this proprietary etched-facet technology (EFT).
These devices have precisely located mirrors with a quality and reflectivity equivalent to those obtained by cleaving. In the EFT process, lasers are fabricated on the wafer in much the same way that integrated circuits are fabricated on silicon. This process allows the lasers to be monolithically integrated with other photonic devices on a single chip and to be tested inexpensively at wafer level (figure 1).
The high process yield, the low cost and the potential for fabrication of integrated GaN photonics make etched-facet blue-emitting lasers very attractive. As a result, BinOptics has developed a modified version of EFT for GaN. To obtain the straight surfaces demanded by photonic devices requires high-quality etching of the semiconductor, but negligible etching of the masking material. The smooth etch quality of the GaN facet can be seen in the SEM image in figure 2.
Reducing defect density
The relatively high defect density in deposited GaN layers has had a strong impact on the yield and cost of blue lasers. GaN grown on sapphire substrates has a typical defect density of 6 x 108 cm-2, primarily resulting from lattice mismatch. A few research labs have developed techniques such as epitaxial lateral overgrowth on sapphire that can reduce defect densities to roughly 107 cm-2, while densities of as low as 3 x 105 cm-2 have been reported for materials grown on very small GaN substrates.
To reduce the problems associated with defects, manufacturers can produce lasers with shorter cavities, which results in fewer defects per device and delivers a much higher device yield. While difficulties associated with cleaving restrict the minimum cavity length to about 600 μm, facet etching allows much shorter cavities of 100 μm. Lasers of this size have a lower maximum power rating due to the shorter cavity, but because most will be used in next-generation DVD-ROM applications, a relatively low output power is acceptable. The specific fabrication, integration and full wafer-testing capabilities enabled by EFT also deliver significant benefits to the fabrication of higher-power GaN lasers for writable optical-disc applications.
BinOptics is currently shipping EFT-manufactured InP-based lasers to several customers. The design flexibility offered by EFT can deliver products with exceptionally high power outputs and efficiencies, and low threshold currents. Device reliability has been proved by accelerated-ageing tests, lasting many thousands of hours, in packaged and non-hermetic conditions.
BinOptics has also demonstrated a novel and cost-effective approach to building a horizontal-cavity surface-emitting laser (HCSEL) using EFT. This is a semiconductor laser with an elongated cavity (on a substrate) that is fabricated by etching a 45° angled facet at the emitter end and a 90° facet at the back end of the cavity (figure 3). The rear reflective region can incorporate an etched distributed Bragg reflector next to the rear facet, or dielectric coatings can be used for reflectivity control. Monitoring photodetectors and receive detectors can also be integrated onto the chip to produce a compact diplexer for two-way communication in access networks.
Mirroring InP progress
The company s development strategy for blue lasers is similar to that used for its InP-based devices. It started by demonstrating effective facet etching with adequate surface quality and selectivity in GaN (see figure 2). This was followed by fabricating edge-emitting blue-laser chips with etched facets - initially Fabry-Pérot ridge waveguide lasers emitting at 405 nm for optical-storage applications. BinOptics has already demonstrated a process to produce this type of structure (see figure 4) and its efforts are now directed at optimizing the device s optical and electrical properties. The next stage is to demonstrate a surface-emitting blue laser using the HCSEL design.
If surface-emitting lasers can be fabricated, this could lead to two-dimensional arrays for high-power applications and monolithic integration of additional functions. For example, an HCSEL could be integrated with receive detectors to create a compact optical head. BinOptics intends to develop these lasers and integrated devices for other applications that require a wavelength accessible with GaN-based material. The company closed a $10 million round of financing in February that will partly fund this development.
Potential market impact
In addition to their use in next-generation DVD players (high-definition DVD and Blu-ray), blue semiconductor lasers are key components in other emerging applications, including the detection of biological and chemical weapons, pollution monitoring, projection displays and high-quality laser printers. According to market analyst Asif Anwar of Strategy Analytics, growth in all of these sectors will drive the market for 405 nm lasers from $9 million in 2003 to $272 million in 2009, although the data-storage market will dominate.
GaN-based EFT could play a major role in driving this increase in sales through increased device yields, chip-size reduction and functional integration. Affordable blue lasers will accelerate the adoption of the emerging high-density-disc standards and open up additional high-volume market opportunities.
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